The present invention relates to improvements in or relating to call control and is more particularly, although not exclusively, concerned with call admission.
In traditional telephony, that is, circuit switched telephony, for a call to be established between two remote telephones, that is, telephones connected to different local exchanges, signalling is used to establish a path prior to establishing the call itself. The path in the above example comprises initiating telephone to its local exchange, initiating local exchange to trunk connection, trunk connection to receiving local exchange, and receiving local exchange to receiving telephone. Here, the signalling and the call usually take the same path and there is full control of the path through each element in the path. As there is full control, it is relatively straightforward to determine whether a call between two telephones can be established or not.
In conventional internet protocol (IP) telephony, the local exchanges are replaced by local “gatekeepers” which communicate with one or more trunk gatekeepers to establish the path between the initiating telephone and the receiving telephone. Here, signalling is effected through the trunk gatekeeper(s) but the call does not take the same path. In this case, the trunk gatekeeper(s) control the bandwidth which can be used in establishing the call, and if the bandwidth is not sufficient, the call is not established.
With the advent of opaque trunk IP telephony, there is no gatekeeper in the IP network which forms the “trunk”. As a result, there is effectively no control over being able to establish a call successfully. Here, the initiating telephone cannot be certain that a call, once established, will be successfully completed.
One known document, “Performance Evaluation of a New End-to-end Measurement Based Call Admission Control Scheme for Supporting IP Telophony” in PROCEEDINGS OF THE SYMPOSIUM OF PERFORMANCE EVALUATION OF COMPUTER AND TELECOMMUNICATION SYSTEMS, 15-19 Jul. 2001 pages 498-505, XP009020891, discloses a receiving gateway, positioned within a network, that is able to identify the gateway from which each frame/packet was sent and to which call each frame/packet belongs. The gateway checks each frame's sequence number to calculate whether any previous packets or frames have been lost, updating a packet loss counter accordingly. At the end of each predetermined measurement period, a control packet is sent to each sending gateway to inform the senders about the loss statistics of their calls. From the statistics reported to the receiving gateway in the control packet(s), the sending gateway can determine whether to reject or accept any new calls to a destination.
Another known document, European Patent Application No. EP 0 932 282 A, discloses an admission control apparatus having a connection request queue. All new connection requests are stored in this connection request queue, which operates on a first-in first-out basis unless admission control is invoked. The admission control detects any packet resends due to packet loss or discard. The admission control derives a pattern of these packet losses or discards, called a packet loss characteristic, to identify any patterns. If certain criteria are met, or the packet loss characteristic matches of pre-determined pattern, admission control is invoked and any new connection requests in the queue will be delayed or discarded, or existing connections will be discarded, until the condition clears.